mpv: Remove waf script from buildInputs.

Since b23dbb1a5dffbfa3abb47fcd0f1579ac2e6f29fc, if buildInputs contains
a plain file it is used as a setup hook. The waf script which is used
here in mpv however isn't a setup hook and also shouldn't be included in
buildInputs as it was kind of a no-op before already.

Failed build log:

https://headcounter.org/hydra/build/582548/nixlog/1/raw

Signed-off-by: aszlig <[e-mail]>

pkgs/applications/video/mpv/default.nix

···
   };
 
   buildInputs = with stdenv.lib;
-    [ waf python3 lua perl freetype pkgconfig ffmpeg libass docutils which libpthreadstubs lua5_sockets ]
+    [ python3 lua perl freetype pkgconfig ffmpeg libass docutils which libpthreadstubs lua5_sockets ]
     ++ optionals x11Support [ libX11 libXext mesa libXxf86vm ]
     ++ optional alsaSupport alsaLib
     ++ optional xvSupport libXv